Hydrogen purifiers are devices that separate hydrogen from hydrogen-rich gaseous mixtures, providing a pure stream of hydrogen for a variety of uses. Typical applications include supplying hydrogen for fuel cells from reformed gases, purifying commercial grade hydrogen to provide for ultra-pure hydrogen used for semiconductor processing, supplying hydrogen for the food industry, purifying hydrogen from an electrolysis stream for laboratory uses, and many other industrial applications.
Typically hydrogen purifiers utilize a thin, hydrogen-permeable metal membrane to effectively separate hydrogen from a gaseous mixture containing hydrogen. While there are a variety of alloys, the most commonly used alloys are Pd77Ag23 and Pd60Cu40. These alloys are rolled into foils that are approximately 25 μm thick, and are then incorporated into a hydrogen purifier. Thinner foils may be used, but they are more fragile and prone to pinhole leaks, which degrades the purity of the supplied hydrogen.
A purifier may ideally operate between about 250° C. and 700° C. depending on the operating condition requirements, the material construction constraints of the purifier, and the gases introduced. For example, in a methanol steam reformer, the hydrogen rich gas produced by the reforming reaction will require a purifier operational temperature of at least about 300° C., in order to reduce the deleterious effects of carbon monoxide at the membrane surface. In other cases the reformer may use a different fuel, such as natural gas, in which the steam reforming temperature will be much higher, such as 550° C.-700° C. These different constraints will require different materials; for operation at 300-400° C. Pd60Cu40 will work well, but above about 400° C. Pd77Ag23 may be preferred due to its superior durability at higher temperatures. In either case, steam reforming with subsequent hydrogen separation utilizing a purifier will typically require a gas pressure in the range of 5-20 atmospheres on the high pressure side of the membrane for effective operation of the purifier. The permeate side pressure will vary depending on the output flux according to Sievart's law. Further, in some cases the temperature of the reformed gases must be reduced prior to their introduction to the purifier; it is generally desirable to maintain purifiers below 450° C. to prevent unwanted intermetallic diffusion between the membrane and the membrane support or seals at the perimeter. Conversely, purifiers will generally be operated above 300° C. to reduce the effects of carbon monoxide coverage (blockage) at the palladium membrane surface.
Generally, there are three basic issues which are addressed in the prior art concerning hydrogen purifiers: 1) membrane alloy selection, 2) mechanical support of the membrane, and 3) sealing of the membrane in a purification structure. In some cases the mechanical support and the sealing means are interrelated.
For example, in U.S. Pat. No. 6,183,542 Bossard discloses a foil-based hydrogen purifier where a hydrogen permeable foil, such as PdAg, is bonded between two wire mesh structures. However, the disclosure does not address supporting the structure in the event that a reverse-pressurization of the membrane occurs.
In U.S. Pat. No. 6,613,132 Bossard shows a purification module consisting of coiled tubes which are attached to a collection header. In this embodiment, the tubes are pressurized from the inside and reverse pressurization of the tubes will cause them to collapse.
In other embodiments utilizing tubes, the exterior of the tube is pressurized while the interior of the tube contains a support such as a spring to prevent collapse. While these tubes will handle a certain amount of reverse-pressurization (ballooning), the overall architecture is difficult and expensive to fabricate.
Ogawa et. al show a composite structure for hydrogen purification in U.S. Pat. No. 5,782,960. Here the inventors utilize a foil bonded or laminated to a porous metal member. In the preferred embodiment the invention utilizes plural metallic supports with rectangular openings formed by an etching process. In the invention the supports are etched prior to attachment of the membrane. The described means of bonding or laminating consists of diffusion bonding or brazing. The patent illustrates supporting the membrane when the permeate pressure is lower than the pressure across the opposing face, but does not include means to support the membrane when the permeate pressure is higher.
A similar composite structure with a palladium-alloy integrated with a support screen is illustrated in U.S. Pat. No. 7,144,444 with Takatani et. al. No contemplation is provided for supporting the membrane in the event that it is reverse-pressurized.
In US 2003/0033933, Frost and B. Krueger pursue the patenting process with the membrane separator of Allegheny Technologies. In this and in preceding patents a hydrogen-permeable foil is disposed over a disc with a seal at the center and periphery (formed by welding), where the mechanical support of the membrane is accomplished with the use of a metal mesh. In this particular patent application Frost and Krueger further add a coating over the wire mesh of a nitride, oxide, boride, silicide, carbide, or aluminide to prevent intermetallic diffusion between the mesh and the membrane. The interdiffusion of iron and other elements is known to reduce the hydrogen permeability of the palladium alloy membrane, which as claimed by the inventors is blocked by the coatings. Similar architectures are also disclosed in U.S. Pat. No. 6,602,325, U.S. Pat. Nos. 6,835,232, and 6,582,499, with none of the architectures supporting the membrane in a reverse-pressurization mode.
Juda et. al in U.S. Pat. No. 5,904,754 disclose a perimeter seal with a PdCu membrane, utilizing diffusion bonding with a copper-surfaced metallic frame. However, the disclosure does not illustrate an architecture which supports the membrane in either axis adequately.
In U.S. Pat. No. 7,101,421 Edlund et. al disclose a purifier module. In the embodiments described, a planar membrane resides on a screen support while the hydrogen-containing mixed gas travels across the membrane in a plenum formed by a feed plate/gasket. In the event that the membranes are reverse-pressurized, they will not be supported. In addition, the description does not prevent wrinkling of the membrane as it expands on hydrogen uptake, which can lead to leakage.
Finally, in U.S. Pat. No. 6,183,543, Buxbaum shows a planar membrane with mechanical support on both the mixed gas and permeate sides. However, this embodiment has several weaknesses. First, the design uses perimeter plates with inserted porous elements for supporting the membrane. Since the inserted porous elements will not be an exact fit, there will exist a gap between the perimeter plates and the elements, into which a hydrogen permeable membrane may wrinkle as the foil expands on hydrogen uptake. Further, the edges of the porous elements may in some instances be sharp (as in the case of woven metal screens), which will tend to perforate the membrane at the edges. Therefore, while Buxbaum's design will support the membrane with forward or reverse pressurization, the membrane is likely to fail eventually because it is not supported across the inevitable gap between the perimeter plate and porous supports.
While all of the above examples allow for the fabrication of membrane purifiers, none of them prevents membrane wrinkling on hydrogen uptake while protecting the membrane during reverse-pressurization, and allowing for low-cost manufacturability and simplified sealing. Therefore, an improved architecture is needed.